Known examples of the blow-molded foam include a climate control duct provided in an instrument panel of a vehicle. The climate control duct often employs a foamed duct formed by molding a foamed resin material. The foamed duct is light in weight. Such a foamed duct can be manufactured easily in a manner that, for example, the resin material such as polyolefin resin including a foaming agent is melted and kneaded and then a foamed parison extruded out of a die head (die slit) of an extruder is blow-molded.
The resin material used for the blow-molded foam is often polyolefin resin. In particular, polypropylene resin is generally used. In recent years, using polyethylene resin alternatively has been examined for the purpose of reducing the material cost.
As described above, in blow-molding, molten resin is extruded from the die slit as a cylindrical parison, and is blow-molded in a mold. At this time, thickness of the parison is adjusted by adjusting a die-slit interval (slit width), so that a wall thickness of the hollow molded article to be molded is a designed value.
However, when a size of the hollow molded article is large or the wall thickness thereof is thick, weight per shot is increased to cause possibility of drawdown. Drawdown is a phenomenon in which the parison is stretched by its own weight and the wall thickness of an upper portion thereof is reduced. Drawdown results in wall thickness difference in a product (the hollow molded article).
In order to avoid drawdown, for example, it may also be possible to change a compound ratio of resin to be used. However, it is undesirable because the resin to be used is restricted by this method. Therefore, a method for correcting the thickness of the parison extruded from the die slit has also been proposed (for example, refer to Japanese Patent No. 07-290563).
In particular, Japanese Patent No. 07-290563 discloses a blow-molding method in which a metering stroke S of an accumulator is set based on a required resin weight W calculated from a target length L and a target wall thickness t of the parison. Further, a die gap set value Dt obtained by dividing the target wall thickness t by a swell ratio R is corrected by a wall thickness reduction amount due to a drawdown phenomenon caused by a stroke length x. Thus, a die gap final set value Df is set to form a desired parison.